Advanced ceramic materials are chemically inert compounds with high thermal stability and mechanical strength. Such characteristics make these materials attractive candidates for applications such as heat engines, cutting tools, and turbine blades, articles which are presently made with expensive super alloys. Current interest in advanced ceramics centers around such materials as carbides, nitrides, borides, and silicides which have properties of hardness, corrosion resistance, and thermal stability that cannot be matched by metallic alloys or other structural materials. Examples of these ceramic materials are SiC, Si.sub.3 N.sub.4, TiC, TiN, VC, WC, and BN. Other nitrides and carbonitrides are useful as superconducting materials and include NbN, MoN, and Nb(C,N).
Although chemical inertness of advanced ceramics is in advantage in these applications, it makes fabrication of components through pressing and sintering a difficult task and places stringent demands on the purity and morphology of the starting materials. Previously, these compounds were prepared by a very high temperature reaction in a nitrogen atmosphere using metal oxide or pure metal powder and carbon as reactants. The reaction yielded clumps of product material that had to be ground into a powder before it could be used. Not only are the high temperature reaction and grinding steps difficult and costly processes, they can also be a serious source of contamination.
More recently, attempts have been made to prepare metal nitrides by reacting the transition metal halides with ammonia or a nitrogen and hydrogen gas mixture. The ammonia or nitrogen atmosphere not only provides the reactant for making the nitride, but it also assures the absence of oxygen which can cause damage of the final ceramic product if it is present during the reaction. However, when titanium chloride was reacted with ammonia at 1000.degree. C., the titanium nitride product was in the form of hard clumps that required grinding before it could be used, and there was also a hydrogen chloride by-produce which is reactive and corrosive to the ceramic material.
To further improve this process, a borohydride has been added to the reaction to prevent a reverse reaction of the ammonium halide and allow the reaction to proceed under moderate conditions producing a transition metal haloamide precursor. This precursor, when reacted with a salt to displace the halide, forms a transition metal amide that forms a nitride upon pyrolysis.
Transition metal alkylamides can be pyrolyzed into ceramics, but the process is very inefficient. The starting material has a low metal content and is a liquid that tends to volatilize during pyrolysis. A high metal content precursor is desirable in order to prepare a ceramic powder with maximum efficiency.
Such problems have led researchers to attempt to develop appropriate precursors that can be converted to ceramic materials by chemical means requiring less rigorous conditions and producing a product that is in a more readily usable form. This accomplishment would be an important development in the area of ceramic production.